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Chapter 1

Introduction

A water cooler is a device that cools and supplies water for drinking purposes. They are extensively

used for home as well as commercial applications. For home applications, they are more

commonly referred to as water dispensers, which along with cooling water also purify it. The

cooling capacities range from around 3 L/h to 5 L/h and they consume 100 W to 200 W power for

a family of five to six people. In the latest models, water heating systems are also being coupled

with water coolers to enhance their productivity. For commercial applications the cooling

capacities could range from 40 L/h for small establishments to 200 L/h for large commercial

establishments like hospitals, malls, railway stations, hostels and educational institutions.

(a) (b)

Photo 1. (a) An USHA 3 L/h water dispenser for home applications (b) A 200 L/h large USHA

water cooler for commercial usage

Most of today’s water coolers work on vapour compression cycle. R134a is widely used as the

refrigerant due to its low boiling point (-23oC) and very low ozone layer depletion potential. The

major components of vapour compression cycle are the compressor, condenser, expansion valve

and the evaporator. The diagrammatic representation of a typical single stage vapour compression

cycle is shown in Figure 1.

(a)

(b)

Figure 1. (a) A typical vapour absorption refrigeration cycle (b) Temperature-entropy diagram

of the vapour absorption refrigeration cycle

The temperature-enthalpy diagram of the cycle is shown in Figure 2. The circulating refrigerant

vapour enters the compressor at point 1 (shown in figure 3). In the compressor, the vapour is

compressed at nearly constant entropy and exits it at a higher temperature but still below vapour

pressure at point 2. From point 2 to 3, the vapour cools to the vapour pressure and then enters the

condenser, where it condenses to form liquid refrigerant. Between points 4 and 5, the vapour passes

through the expansion valve, its pressure decreases abruptly at a constant enthalpy. The resultant

liquid-vapour refrigerant enters the evaporator, where it boils by absorbing heat from water,

thereby cooling it.

Chapter 2

Compressors

A refrigeration compressor is a machine used to continuously draw the refrigerant vapour from the

evaporator, so that a low pressure and low temperature can be maintained in the evaporator at

which the refrigerant can boil by extracting heat from the refrigerated space. The compressor then

has to raise the pressure of the refrigerant to a level at which condensation can occur by rejecting

heat to the cooling medium in the condenser. It also continually circulates the refrigerant through

the refrigerating system. The compressor is driven by some prime mover.

2.1 Classification of Compressors

Compressors can be of various different types, based on the fluid flow, the method of compression,

the number of working strokes, the number of stages, the method of drive employed and the

location of prime mover.

2.2.1 Based on the fluid flow

1. Positive displacement compressors

In positive displacement compressors, the fluid flow is intermittent. These compressors

ensure positive admission and delivery of fluid preventing undesired reversal of flow

within the machine.

2. Rotodynamic compressors

In rotodynamic compressors, the flow is continuous. The fluid is subject to flow processes

and the work is transferred by virtue of the change of momentum of stream of fluid flowing

at a high speed over blades or vanes attached to a rotor.

2.2.2 Based on the method of compression

1. Reciprocating compressors

The compressors in which the vapour refrigerant is compressed by the back and forth

motion of the piston are called reciprocating compressors.

2. Rotary compressors

The compressors in which the vapour refrigerant is compressed due to the movement of

blades are called rotary compressors.

3. Centrifugal compressors

The compressors in which rise of pressure of vapour refrigerant is done by a centrifugal

force are called centrifugal compressors.

2.2.3 Based on the number of stages

1. Single-stage compressors

In a single-stage compressors, the vapour refrigerant is compressed in a single cylinder

and then delivered at high pressures. These compressors need a crankshaft seal to prevent

the leakage of refrigerant.

2. Multi-stage compressors

In multi-stage compressors, vapour refrigerant is delivered at high pressures using two or

more cylinders placed in series. In these compressors, use of crankshaft seal is eliminated.

They are used for small capacity refrigeration systems like domestic refrigerators and home

freezers.

2.2.4 Based on the location of prime mover

1. Semi-hermetic compressors

In semi-hermetic compressors, the direct drive, motor and the compressor are all placed

in separate housings.

2. Hermetic compressors

In hermetic compressors, the direct drive, motor and compressor are all placed in the same

housing.

2.2 Reciprocating Compressors

The compressors in which vapour refrigerant is compressed by the reciprocating motion of the

piston are called reciprocating compressors. This compressors are used for refrigerants which have

comparatively low specific volume and large differential pressure such as R17, R12, R22 and R40.

Reciprocating compressors are available in sizes as small as 1/12 KW which are used in domestic

refrigerators and up to about 150 KW for large capacity installations. Two types of reciprocating

compressors in general use are single acting vertical compressor and double acting horizontal

compressor. Single acting compressors have their cylinders arranged vertically, radially or in a V

or W form. Double acting compressors usually have their cylinders arranged horizontally.

Figure 2. Sectional view of a reciprocating compressor

Reciprocating compressors can be of two different types:

Single acting reciprocating compressors

The reciprocating compressors are the compressors in which the compression is

accomplished using only side of the piston.

Double acting reciprocating compressors

The compressors using both sides of the piston during compression are known as double

acting reciprocating compressors.

2.3 Rotary Compressors

Rotary compressors are positive displacement machines in which compression of the fluid is

affected directly by a rotor. Since the clearance in rotary compressors is negligible, therefore they

have high volumetric efficiency. These compressors may be used with refrigerants R-12, R-22, R-

114 and R-17. Rotary compressors can be of three basic types:

Rotating blade type rotary compressors

The rotating blade type rotary compressors consist of a cylinder and a slotted rotor

containing a number of blades. The centre of the rotor is eccentric with the centre of the

cylinder. The blades are forced against the cylinder wall by centrifugal action during the

rotation of the rotor.

Figure 3. Rotating blade type rotary compressor

Screw compressors

Rotary screw compressors consist of two helically grooved rotors which rotate in a

housing. The male rotor consists of lobes and is normally the driving motor. The female

rotor has gullies and is normally the driven rotor. A four lobe male rotor will drive a six

gully female rotor at two third of its speed. As they are high speed rotary machines, a large

volume can be handled by them therefore they are found extremely suitably for large

capacity, low-temperature applications like central air conditioning plants.

Figure 4. Screw compressor

Scroll compressors

Scroll compressors are valve-less positive displacement machines like rotary and screw

compressors. Because of their simplicity, they have become very popular with the industry

in recent years. Here the compression is achieved by two intermitting, spiral shaped scroll

members, one of which is a fixed screw and the other one is an orbiting scroll. The capacity

ranges from 1 to 14 TR.

Figure 5. Scroll compressor

2.4 Centrifugal Compressors

Centrifugal compressors increase the pressure of low pressure vapour refrigerant to high pressure

by centrifugal force. The centrifugal compressor is generally used for refrigerants that require large

displacement and condensing pressure such as R11, R113. A simple centrifugal compressor

consists of an impeller to which a number of curved vanes are fitted symmetrically. The impeller

rotates in an airtight volute casing with inlet and outlet points.

Figure 6. Centrifugal compressor

2.5 Selection

The compressor chosen for the desired application, i.e. a medium capacity water cooler is a single-

acting, reciprocating type, single-stage, hermetic sealed compressor.

The hermetic, reciprocating type compressor has been chosen for the desired application because

it offers several advantages as compared to its other counterparts. These are as follows:

Hermetic sealed compressors prevent the leakage of refrigerant.

Hermetic compressors require small space because of their compactness.

The lubrication of hermetic compressors is simple as the motor and compressor operates

in a sealed space with the lubricating oil.

Hermetic compressors are less noisy.

Reciprocating compressors are available in sizes as small as 1/12 KW which are used in

small domestic refrigerators and up to about 150KW.

Centrifugal compressors are not practical below 50TR capacity.

Manufacturing cost is high for centrifugal compressors.

Power requirement is lower for reciprocating compressors as compared to centrifugal

compressors.

The cost of a typical rotary screw compressor, in terms of initial purchase price and

installation, is generally less than a reciprocating compressor. However, provided that it is

properly maintained, a reciprocating compressor can be expected to last between two and

five times longer.

Chapter 3

Condensers

Condenser is a heat exchanger which removes heat of the hot refrigerant discharged from the

compressor, thus condensing the vapour refrigerant in the process. Condensation is the change of

the physical state of matter from gas phase into liquid phase. Condensation processes can be

classified into two types:

Film wise condensation

Drop wise condensation

Compared to film-wise condensation, drop-wise condensation has a greater surface heat-transfer

coefficient as it has a greater area exposed to the saturation vapour. Hence the design of the

condenser changes with the type of condensation phenomenon occurring too.

The hot vapour refrigerant which is condensed consists of the heat absorbed by the evaporator and

the heat of compression added by the mechanical energy of the compressor motor. The heat from

the hot vapour refrigerant is removed first by transferring it to the walls of the condenser tubes and

then from the tubes to the condensing medium, which may be air or water or a combination of

both.

3.1 Stages in Condensation

Condensation in a condenser occurs in three stages:

De-superheating of the hot gas.

Condensing of the gas to liquid state and release of the latent heat.

Sub-cooling of the liquid refrigerant.

Since the phase in each stage is different, the design of each stage must be taken separately.

Generally sub-cooling part in the condensers is very less and is neglected. But in systems with

very high sub-cooling, sub-cooling cannot be neglected.

Figure. Temperature profile during condensation

3.2 Classification of Condensers

Condensers are classified on the basis of type of cooling method employed. Application wise

condensers are widely used in HVAC-R industry and hence further discussion on condensers is

carried out in the context of HVAC-R industry. Hence condensers are classified into air cooled,

water cooled, and evaporative condensers.

3.2.1 Water cooled condenser

In a water cooled condenser, the heat rejected in all of the stages of the condensation is received

as sensible heat by the water which is acting as coolant. Water cooled condensers require regular

maintenance due to scaling. Due to large heat transfer coefficient of water the area required for

heat transfer is less. Hence where there is no scarcity of water and space is a constraint, water

cooled condensers are preferred. A water cooled condenser can be shell and coil condenser, tube

in tube condenser, or shell and tube condenser.

3.2.1.1 Shell and coil condenser

This type of condenser is used for medium capacity refrigeration. It is a variant or rather simpler

version of shell and tube condenser. The tubes are replaced by a spiral coil. The coil is inserted in

the shell and according to the capacity requirement, the refrigerant can flow inside or outside the

coil. Finned coils can also be used to increase the rate of heat transfer.

Figure. A shell and coil condenser

3.2.1.2 Tube in tube/Double tube condenser

This type of condenser is used for low capacity refrigeration up to 10 TR. The refrigerant flows in

the annulus. Long tubes cannot be used as it increases the pressure drop on the refrigerant side.

Hence long tubes are not suitable for large capacity condensers.

Figure. A tube in tube condenser

3.2.1.3 Shell and tube condenser

This is the most common type of condenser used in systems from 2 TR up to thousands of TR

capacity. In these condensers the refrigerant flows through the shell while water flows through the

tubes in single to four passes. The condensed refrigerant collects at the bottom of the shell. The

coldest water contacts the liquid refrigerant so that some sub cooling can also be obtained.

Figure. A schematic of a shell and tube type condenser

3.2.2 Air cooled condenser

In an air cooled condenser the secondary fluid is air. Due to lesser heat transfer coefficient as

compared to water, a larger heat transfer area is required. Air cooled condenser is widely used in

commercial refrigeration system with low to medium capacity. An air cooled condenser can be of

natural convection type or forced convection type.

3.2.2.1 Natural convection type

Natural convection type condensers are used for small capacity units like household refrigerators

and freezers. No power is required to run these type of condensers. The heat transfer coefficient in

this type is very less when compared to forced type. Hence a large heat transfer area and thereby

a larger length of the condenser is required.

Figure. A natural convection type condenser

3.2.2.2 Forced convection type

In a forced convection type condenser, the circulation of air over the condenser surface is

maintained by using a fan or a blower. These condensers normally use fins on air-side for good

heat transfer. Forced convection type condensers are commonly used in window air conditioners,

water coolers and packaged air conditioning plants. An evaporative condenser uses the evaporation

of water spray to remove the latent heat of condensation of the refrigerant during condensation. It

is actually a simplified combination of a water cooled condenser and cooling tower.

Figure. A forced convection type condenser

3.2.3 Evaporative condenser

Evaporative condensers are used in medium to large capacity systems. These are normally cheaper

compared to water cooled condensers, which require a separate cooling tower. Evaporative

condensers are used in places where water is scarce. Water is used in a closed loop in this type of

condenser. Since condenser has to be kept outside, this type of condenser requires a longer length

of refrigerant tubing, which calls for larger refrigerant inventory and higher pressure drops.

Figure. A diagram of evaporative condenser

3.3 Selection

For the required application, design of both water cooled and air cooled condensers will be carried

out. Among air-cooled condensers, the forced convection type condenser is selected because the

natural convection type has larger space requirements. Among the water-cooled condensers, the

shell and coil type condenser is selected for design because shell and tube type requires frequent

maintenance due to high fouling rate, whereas tube and tube type requires a very large length,

which increases the pressure losses beyond the permissible limits, which is undesirable.

Chapter 4

Evaporators

An evaporator is a heat exchanger in which refrigerant is evaporated, thereby cooling a fluid (here

water) circulating through the evaporator. The latent heat absorbed by the refrigerant cools the

water around it.

4.1 Boiling in Evaporators

Boiling is a liquid-to-vapour phase change just like evaporation, but there are significant

differences between the two. Evaporation occurs at the liquid-vapour interface when the pressure

is less than the saturation pressure of the liquid at the given temperature. Boiling, on the other

hand, occurs at the solid-liquid interface when the liquid is brought into contact with a surface

maintained at a temperature Ts sufficiently above the saturation temperature of the Tsat of the liquid.

Boiling is classified as pool boiling or flow boiling, depending on the presence bulk fluid motion.

4.1.1 Pool boiling

In pool boiling, the fluid is not forced to flow by a mover such as a pump, and any motion of the

fluid is due to natural convection currents and the motion of bubbles under the influence of

buoyancy.

Boiling regimes and boiling curve

During boiling four different regimes are observed: natural convection boiling, nucleate boiling,

transition boiling and film boiling.

1. Natural Convection Boiling (up to point A): During this stage the liquid is slightly

superheated and evaporates as it rises to the free surface. The motion of the fluid in this

case is purely due to natural convection currents.

2. Nucleate Boiling (between points A and C): The first bubbles start forming at A of the

boiling curve at various preferential sites of the heating surface. Point A is called onset of

nucleate boiling.

3. Transition boiling (between points C and D): As the heater temperature is increased beyond

C, the heat flux decreases. In transition boiling, both nucleate as well as film boiling occur.

4. Film boiling (beyond point D): In this region the heat transfer is completely covered by a

continuous vapour film. The heat transfer coefficients are low in this regime.

Figure 4.1. Various regimes obtained during boiling

4.1.1 Flow Boiling

In flow boiling, the fluid is forced to move by an external source such as a pump as it undergoes a

phase change process. The boiling in this case exhibits the combined effects of convection and

pool boiling. The flow boiling can also be classified as either external flow boiling or internal flow

boiling.

External flow boiling over a plate or cylinder is similar to pool boiling, but the added

motion increases both the nucleate boiling heat flux and the critical heat flux considerably.

Internal flow boiling is much more complicated in nature because there is no free surface

for the vapour to escape, and thus both the liquid and the vapour are forced to flow together.

The two-phase flow in a tube exhibits different flow boiling regimes, depending on the

relative amounts of the liquid and the vapour phases. This complicates the analysis even

further.

4.2 Classification

Evaporators are primarily classified on the basis of the type of construction, on the basis of

refrigerant circulation and the mode of heat transfer. This classification is further discussed in the

following sections.

4.2.1 Based on the type of construction

Bare-Tube-Coil Evaporators

They were one of the first types of evaporators used. They consist of long run of coils,

through which the refrigerant flows. They are simple in construction and hence can be

easily fabricated and cleaned. However, they provide a very little contact area and hence

the length of the tube has to be very long even for low capacity refrigeration systems. They

are usually constructed of either steel pipe or copper tubing. Steel pipes are used for large

evaporators while copper tubing is for smaller evaporators. Common shapes are flat, oval

trombone and spiral.

(a) (b)

Figure 1. Common designs of bare tube evaporators. (a) Flat zigzag coil (b) Oval trombone

Finned-Tube Evaporator

These consist of bare tubes or coils over which metal plates or fins are fastened. The fins,

serving as secondary heat absorbing surfaces, have the effect of increasing the overall

surface are of the evaporator, thereby improving its efficiency. The metal fins are

constructed of thin sheets of metal having good thermal conductivity. Fin spacing is kept

large for larger tubes and small for smaller tubes. Formation of frost drastically reduces

their performance, as the frost restricts the passage of fluid between the fins. Hence, they

should never be used for refrigeration temperatures below 0oC. They are primarily

designed for air-conditioning applications.

Figure 2. Actual Finned Tube Evaporator

Plate-Surface Evaporator

These evaporators consist of formed tubing installed between two metal plates, which are

welded together at edges. In order to provide good thermal contact between the welded

plates and the tubing carrying is evacuated so that the pressure of the atmosphere exerted

on the outside surface of plates, holds the plates firmly against the tubing inside. These are

widely used as partitions in freezers, frozen food display cases, ice-cream cabinets, etc.

They are helpful when unusual peak load conditions are experienced frequently.

Figure 3. Standard Serpentine Plate Evaporator

Shell-and-Tube Evaporator

The shell-and-tube evaporators are very efficient, require minimum floor space and head

space. They are easy to maintain, and hence are widely used in medium to large capacity

refrigeration systems. They consist of a shell and a large number of tubes arranged parallel

to each other. The shell diameters range from 150 mm to 1.5 m. The number of tubes may

vary from fifty to several thousand, each of length 1.5 m to 6 m. A series of baffles channels

the fluid throughout the shell side. The baffles create cross flow through the tube bundle

and increase the velocity of fluid, thereby increasing its heat transfer coefficient. The

velocity of fluid flowing perpendicular to the tubes should be at least 0.6 m/s to clean the

tubes and less than the velocity limit of the tube and baffle materials, to prevent erosion.

Figure 5. An actual shell and tube type evaporator

Shell-and-Coil Evaporator

These evaporators consist of a tank in which the fluid to be cooled is placed. A simple

coiled tube is used for cooling the fluid. The fluid in shell gives large amount of thermal

storage capacity and hence has can give good performance with small but infrequent peak

loads. Sometimes, the refrigerant is placed inside the shell and the fluid to be cooled flows

through the tube. These types of shell-and-coil evaporators can give instantaneous cooling.

They are mostly used at low capacities, e.g., for bakeries, photographic labs and to cool

drinking water.

Figure 7. A shell and coil type evaporator

Baudelot Evaporator

These evaporators are used to cool the fluid to near its freezing temperature. The fluid

circulates over the outside of vertical plates, which are easy to clean. The inside surface

of the plates is cooled by evaporating the refrigerant. The fluid to be cooled flows

uniformly from the top of the evaporator to the bottom. Insulation is usually provided to

prevent heat loss. They are mainly used for industrial applications and especially in dairy

industry.

Figure 8. A Baudelot evaporator

4.2.2 Based on refrigerant circulation

Direct-Expansion Evaporator

The refrigerant evaporates inside the tubes of this evaporator. They are usually used with

positive displacement compressors like reciprocating compressors, rotary screw

compressors etc. The distribution of refrigerant is critical in these types of evaporators, as

any imbalance causes a drastic decrease in heat transfer coefficient. The number of passes

can be single or multiple. They are generally used for low to medium capacity refrigeration

applications.

Figure. Direct-Expansion shell and tube type evaporator

Flooded Evaporator

The flooded type of evaporator is always completely filled with the refrigerant. The

evaporated vapour is taken away by the compressor. The inside surface of these

evaporators is always completely wetted with liquids and hence they have very high heat

transfer coefficient. However, they are bulky and require a large amount of refrigerant

charge. One variation of flooded shell-and-tube evaporator is spray shell-and-tube

evaporator. When the refrigerant pressure has an adverse effect on heat transfer coefficient,

the liquid is sprayed in the tubes, rather than flooding them.

Figure. Flooded shell and tube type evaporator

4.2.3 Based on the mode of heat transfer

Forced convection evaporators

A fan or a pump is used to circulate the fluid being refrigerated. As the fluid moves at a

high velocity over the heat transfer surface, the heat transfer coefficient is quite high.

However they cost more and are bulkier.

Natural convection evaporators

The fluid being refrigerated flows by natural convective currents set due to density

difference that arises due to variation in temperature. They are generally used for home and

medium applications, as the heat transfer coefficient is less, but they are cheaper and less

bulky.

4.3 Selection

For the required application, which is a medium capacity water cooler, shell and coil type

evaporator is chosen. The reasons which dictate the choice of shell and coil type evaporator are:

Since only one coil of larger diameter is used, the construction is easier. The single coiled

tube has only to be soldered inside the shell at opening and exit.

The coiled tubing is free to expand and contract with temperature changes because of its

spring action.

The coil can withstand strain caused by temperature changes.

It is suitable for small capacity liquid cooling of around 2-10 TR.

Shell and coil type evaporator is specifically used in applications where cleanliness is

required, e.g. bakeries, photographic laboratories, drinking water, etc. Since the application

involves drinking water which should not be contaminated while being refrigerated, this

type is thus chosen.

Chapter 5

Expansion Devices

Expansion devices are the devices which are used to lower the pressure of the refrigerant from the

high value at the compressor, condenser side to the low value at the evaporator side. There are

basically two functions of expansion devices:

To reduce the pressure from the condenser pressure to evaporator pressure.

To regulate the refrigerant flow rate from the high pressure liquid line into the evaporator

at a rate equal to the evaporation rate in the evaporator.

In cases where constant evaporator temperature is to be maintained, or where it is desired that no

liquid refrigerant should enter the compressor, the mass flow rate has to be controlled in a manner

that only superheated vapour exits the evaporator. It is therefore, required that this control be done

in such a way that minimum energy is used, while meeting the criteria of temperature and cooling

load.

5.1 Classification

The expansion devices which are employed in refrigeration systems can be divided into two broad

categories:

Fixed opening type, in which the flow area remains fixed. These include capillary tubes.

Variable opening type, in which the flow area changes with the changing mass flow rates.

These include Constant Pressure Automatic Expansion Valves, Thermostatic Expansion

Valves and Float type Expansion Valves.

5.1.1 Capillary Tube

Capillary tube is one of the most widely used throttling devices in the refrigeration systems. The

capillary tube is a long copper tube of very small internal diameter coiled to several turns so that

it would occupy less space. The internal diameter varies from 0.5 to 2.28 mm. Capillary tube used

as the throttling device in the domestic refrigerators, deep freezers, water coolers and air

conditioners.

Figure. A capillary tube

5.1.2 Constant Pressure Automatic Expansion Valve

The constant pressure expansion valve keeps the pressure inside the evaporator constant no matter

what the load inside the evaporator be. It does not allow the control of the flow of the refrigerant

through it even when lesser or more refrigerant is required in the evaporator.

The constant pressure valve is also called as the automatic expansion valve but this is the misnomer

since it is not automatic for it does not allow automatic control of the flow of refrigerant through

it. In the very earlier days only hand operated expansion valve or throttling valve was available,

however since this valve allowed automatic constant pressure in the evaporator, it was given the

name automatic expansion valve.

Figure. A schematic of constant pressure automatic expansion valve

5.1.3 Thermostatic Expansion Valve

The thermostatic expansion valve is the automatic valve that maintains proper flow of the

refrigerant in the evaporator as per the load inside the evaporator. If the load inside the evaporator

is higher it allows the increase in flow of the refrigerant and when the load reduces it allows the

reduction in the flow of the refrigerant. This leads to highly efficient working of the compressor

and the whole refrigeration and the air conditioning plant. The thermostatic expansion valve also

prevents the flooding of the refrigerant to the compressor ensuring that the plant would run safely

without any risk of breakage of the compressor due to compression of the liquid.

Figure. A thermostatic expansion valve

5.1.4 Float Valve

Float type expansion valves are normally used with flooded evaporators in large capacity

refrigeration systems. A float type valve opens or closes depending upon the liquid level as sensed

by a buoyant member, called as float. The float could take the form of a hollow metal or plastic

ball, a hollow cylinder or a pan. Thus the float valve always maintains a constant liquid level in a

chamber called as float chamber. Depending upon the location of the float chamber, a float type

expansion valve can be either a low-side float valve or a high-side float valve.

Figure. A sectional view of a Float Valve

5.1.4.1 Low Side Float Valve

A low-side float valve maintains a constant liquid level in a flooded evaporator or a float chamber

attached to the evaporator. When the load on the system increases, more amount of refrigerant

evaporates from the evaporator. As a result, the refrigerant liquid level in the evaporator or the

low-side float chamber drops momentarily.

5.1.4.2 High Side Float Valve

A high-side float valve maintains the liquid level constant in a float chamber that is connected to

the condenser on the high pressure side. When the load increases, more amount of refrigerant

evaporates and condenses. As a result, the liquid level in the float chamber rises momentarily.

5.2 Selection

For the medium capacity water cooler application, expansion valve selection is based on the type

of evaporator employed. A table showing different types of evaporators for various capacity ranges

and the expansion valves employed for the corresponding evaporator types. It can be seen that for

the required application, since power requirement is less than 10 kW, a shell and coil type

evaporator in conjunction with a thermostatic expansion valve is used.

A thermostatic expansion valve provides several advantages which make its use suitable. Some of

them are listed as follows:

It maintains the flow of the refrigerant to the evaporator as per the load inside. Thus the

refrigeration or the air conditioning plant can run to the optimum capacity as per the

requirements.

It keeps the evaporator fully active and helps getting the optimum cooling effect from it.

Since the entire refrigerant in the evaporator gets vaporized the chances of the liquid

refrigerant particles going to the compressor are reduced. This reduces the chances of the

breakdown of the compressor due to compression of the liquid.

Table. Summary of characteristics of various types of evaporators for different power capacities

and the corresponding expansion devices used

Type of

Evaporator Subtype Usual Expansion Device

Capacity Range

(kW)

Direct-Expansion

Shell-and-Tube

Thermal Expansion Valve 7-3500

Electronic Modulation

Valve 7-3500

Brazed Plate Thermal Expansion Valve 2-700

Semi Welded

Plate Thermal Expansion Valve 175-7000

Flooded

Shell-and-Tube Low Pressure Float 90-7000

High Pressure Float 90-21100

Spray shell-tube Low Pressure Float 180-35000

High Pressure Float 180-35000

Brazed-Plate Low Pressure Float 2-700

Semi Welded

Plate Low Pressure Float 175-7000

Baudelot Flooded Low Pressure Float 35-350

Direct-Expansion Thermal Expansion Valve 18-90

Shell-and-Coil - Thermal Expansion Valve 7-35

Chapter 6

Design procedure of a water cooler

Design of a water cooler involves design of various components, i.e. compressor, condenser,

evaporator and expansion device. During the study of these components, we have chosen the types

of these components. Now, the design of these components will follow.

6.1 Initial procedure

The water cooler design would be based on Vapour Compression Refrigeration System. The sub

cooling is assumed to be negligible. R134a is selected as the refrigerant.

The temperature reduction and capacity of the water cooler required are assumed. The temperature

of evaporation and condensation are also assumed initially. The compression process is assumed

to be isentropic, and heat losses are assumed to be negligible.

With these assumptions, the Refrigeration Effect can be obtained. Dividing the energy required to

be removed from the water by the enthalpy difference across the evaporator will yield the mass

flow rate of the refrigerant. This mass flow rate can then be used for further design of individual

components of the system.

6.2 Design procedure for reciprocating compressor

The design of a reciprocating compressor for water coolers can be carried out in the following

steps:

1. Find out the index of compression and expansion (both assumed to be same), using the

equation:

k = ln (

pspd)

ln(vdvs)

where ps is the suction pressure in Pa, pd is the discharge pressure in Pa, vs is the

suction volume in m3/kg and vd is the discharge volume, m3/kg.

2. Find out specific compressor work as:

w = psvs (k

k − 1) [(

pdps)

k−1k

− 1]

3. Assume an appropriate clearance ratio (clearance ratio is less than 5% for well-designed

compressors):

ε = vcvsw

where vc is the clearance volume and vsw is the swept volume.

4. Find out the volumetric efficiency as:

ηv = 1 + ε − ε (pdps)

1k

5. Find out the compressor power consumption as:

W = m × w

where m is the mass flow rate of the refrigerant and w is the compressor work.

6. Find out volumetric flow rate of compressor as:

Vr = m× vs

7. Find out required compressor displacement rate

Vs =Vrηv

where Vr is the volumetric flow rate of the refrigerant through the compressor, and ηv is

the volumetric efficiency of the compressor.

8. Assume some stroke to bore ratio, θ based on the following guidelines:

a. Vacuum pumps and high speed air compressors: θ < 0.5

b. Fluorocarbon compressors: θ = 0.8

c. Ammonia compressors: θ = 1.0

d. High pressure compressors: θ = 4.6

9. Assume suitable mean piston speed (1.5 m/s to 5 m/s) and find out rotational speed of

compressor as:

N = 30CmL

where Cm is the mean piston speed in m/s, N is the rotational speed of crank in rad/s, D

is the diameter of the compressor cylinder in m, L is the length of compressor cylinder in

m.

10. Find out bore of the cylinder (assuming single cylinder) as:

D = (240 × Vsπ × θ × N

)

1/3

11. Find out the length of the cylinder as:

L = θ × D

6.3 Design procedure for condenser

Two types of condensers are to be designed – air cooled and water cooled type.

The heat transfer in condenser can be obtained using the following steady state equation:

Q = UA∆Tm

where Q is the total heat transfer rate in W, ∆Tm is the logarithmic mean temperature difference in

K, A is the heat transfer area associated withU in m2, U is the overall heat transfer coefficient in

W/m2K.

Calculation of ∆𝐓𝐦

There are three stages in condensation viz. desuperheating, condensation, sub cooling. Neglecting

the amount of sub cooling, an average Tmis to be found to approximate that condensation takes

place all over the length of condenser.

If Q is the total amount of heat removed in condensation process,

Q = Cpw(Twc − Twi)

For desuperheating zone, Qsv = mrCpv(Tri − Thi)

Twd = Twi +Qsv

mwCpw

LMTDdesuperheat =(Tri − Twi) − (Thi − Twd)

ln(Tri − Twi)(Thi − Twd)

For condensation zone, Qc = mr(hti − hto)

Twc = Twd +Qc

mwCpw

LMTDdesuperheat =(Thi − Twc) − (Tho − Twd)

ln(Thi − Twc)(Tho − Twd)

∆Tm =Qc + Qsv

Σ (Q

LMTD)

Where mwis mass flow rate of coolant stream, Cpwis heat capacity of coolant stream, Qcis heat

released during condensation, Qsvis heat released during desuperheating, Twiis inlet temperature

of coolant stream, Twdis temperature of coolant steam after desuperheating, Twcis temperature of

coolant stream after condensation, Triis temperature of the refrigerant before desuperheating, Thiis

temperature of refrigerant before condensation, Thois temperature of refrigerant after

condensation.

6.3.1 Forced convection air-cooled condensor

The overall heat transfer coefficient for a finned condenser is formulated as,

U0 =1

(AoAihi

) + (AoAirfw) + (

Ao

Aitk) + (

1hr(Ab + Afηf)

)

Where Ao is the outer surface area , Ai is the internal surface area,hi is heat transfer coefficient of

refrigerant, rfwis the resistance due to fouling, t is the thickness of tube, k is thermal conductivity

of metal, hr is heat transfer coefficient of air, Ab is bare area of tube, Af is area of fin, ηfis efficiency

of fin.

The refrigerant side heat transfer coefficient can be estimated from one of the following

correlations:

Chaddock and Chato’s correlation

The average value of heat transfer coefficient over the legth of condenser is estimated as

ho = 0.555 [kf3ρf(ρf − ρg)gh′fg

DiµfΔt]

Where the modified enthalpy of evaporation is defined as, h′fg = hfg + 3CpfΔt/8 , kf is the

conductivity of fluid refrigerant, ρf is density of fluid refrigerant at saturation temperature, ρg is

density of gaseous refrigerant at dry saturation temperature, gis acceleration due to gravity, Diis

internal diameter of the tube, µf is viscosity of fluid refrigerant at saturation temperature.

Shah’s correlation

This correlation takes into consideration the effect of pressure of the refrigerant and also the

quality of mixture. This correlation can also be used for calculating local condensation heat

transfer coefficient. This equation is much similar to Dittus – Boelter equation.

hi = hl [(1 − x)0.8 +3.8x0.76(1 − x)0.04

pr0.38 ]

hl = 0.023(Ref)0.8(Prf)

0.4Kf

Di

pr =p

pcritical

Where Diis internal diameter of the tube, x is dryness fraction, Ref is Reynold number of fluid

refrigerant, Prf is Prandtl number of fluid at saturation temperature, Kf is the conductivity of

fluid refrigerant, p is pressure

Akers, Dean and Crosser relation

This equation is used for turbulent flow of refrigerant inside the tubes.

hl = 5.06(Rem)0.333(Prf)

0.333Kf

Di∶ Reg < 5X104

hl = 0.0265(Rem)0.8(Prf)

0.333Kf

Di∶ Reg > 5X104

Rem = Ref [1 + (ρfρg)

0.5

]

Where Diis internal diameter of the tube, x is dryness fraction, Ref is Reynold number of fluid

refrigerant, Prfis prandtl number of fluid at saturation temperature, Kf is the conductivity of fluid

refrigerant, Reg is Reynold number of gaseous refrigerant,ρf is density of fluid refrigerant at

saturation temperature, ρg is density of gaseous refrigerant at dry saturation temperature.

6.3.2 Water cooled condenser

The overall heat transfer coefficient for shell and tube heat exchanger with refrigerant flowing

outside the tubes and water flowing inside the tubes,

U0 =1

(AoAihi

) + (AoAirfw) + (

AoAm

tk) + (

1hrϕs

)

Where Ao is the outer surface area , Ai is the internal surface area,hi is heat transfer coefficient of

refrigerant, rfwis the resistance due to fouling, t is the thickness of tube, k is thermal conductivity

of metal, hr is heat transfer coefficient of water, ϕsis surface efficiency of fin

Now, heat transfer coefficient on the water side is given by,

hwD

k= 0.023 (

DG

µ)0.8

(cpµ

k)0.4

Where hwis heat transfer coefficient of water, D is hydraulic diameter of the tube, kis thermal

conductivity of water, Gis density of water, µ is viscosity of water, cpis specific heat of water.

6.4 Design procedure for evaporator

The heat transfer in evaporator can be obtained using the following steady state equation:

Q = UA∆Tm

where Q is the total heat transfer rate in W, ∆Tm is the mean temperature difference in K, A is the

heat transfer area associated withU in m2, U is the overall heat transfer coefficient in W/m2K.

6.4.1 Calculation of overall heat transfer coefficient, U

The overall heat transfer coefficient can be either calculated based on the inner surface area or the

outer surface area.

Based on inner surface area

U =1

1hi+

AiAoho

+tk(AiAm

) + rfi

Based on outer surface area

U =1

1ho

+AoAihi

+tk(AoAm

) + rfo

where hi = inside heat transfer coefficient based on inside surface area in W/m2K

ho= outside heat transfer coefficient based on inside surface area, W/m2K

Ai= inside heat transfer surface area, m2

Ao = outside heat transfer surface area, m2

Am= mean heat transfer surface area, m2

k= thermal conductivity of the heat transfer material, W/m.K

t = thickness of heat transfer surface (tube wall thickness), m

rfi = fouling factor of fluid side based on inside surface area, (m2K)/W

rfo = fouling factor of fluid side based on outside surface area, (m2K)/W

6.4.2 Calculation of ∆𝐓𝐦

The value of ∆Tm can be found using the log mean temperature difference

∆Tm = F(T1 − Te) − (T2 − Te)

ln (T1 − TeT2 − Te

)

where

T1 = inlet temperature of fluid, K

T2 = outlet temperature of fluid, K

Te = evaporator temperature, K

F = correction factor for cross flow and parallel flow

6.4.3 Calculation of convective heat transfer coefficient h

6.4.3.1 Heat transfer coefficient on refrigerant side

The refrigerant boils inside the evaporator while flowing. Hence it can be considered to be an

aggregate of nucleate pool boiling and forced convection. Heat flux due to nucleate pool boiling

is calculated using pool boiling equation, while that due to forced convection can be found using

standard forced convection equations.

Nucleate Pool Boiling Relation

Rohsenows’ Relation: This correlation is applicable to clean surfaces and is fairly independent of

geometry of the surface

q = µlhfg√g(ρl − ρv)

σ(

cpl∆Te

CsfhfgPrn)

3

The corresponding heat transfer coefficient can be found using the relation

q = h(Ts − Te)

Forced Convection Relations

Dittus and Boelter Equation: This equation gives heat transfer coefficient for forced convection

Nu = 0.019(Rel)0.8Pr0.3

Once heat transfer coefficients are found from the above two equations, the total heat transfer

coefficient can be found by adding them.

T1

T2

Te

T

x

Combined Flow Boiling Equations:

Bo-Pierre’s Equation: This equation can be used when the inlet dryness is greater than 0.1 and

less than 0.16

Nu = 0.0082(Re2Kf)0.5

Kf is called load factor, given by the equation

Kf =∆xhfg

L

Legend for conventions used in this section are:

q = heat flux, W/m2

µl = viscosity of refrigerant, Pa.s

hfg = latent heat of vaporization of refrigerant, J/kg

g = acceleration due to gravity, m2/s

ρl = density of liquid refrigerant, kg/m3

ρv = density of vapour refrigerant, kg/m3

σ = surface tension, N/m

cpl = specific heat of refrigerant, J/kgK

∆Te = Ts − Te

Ts= surface temperature of tubing, K

Te = evaporation temperature of the refrigerant, K

Pr = Prandtl number of the refrigerant

n = a constant (usually 1.7)

Csf = constant, 0.013 for halocarbons

Nu= Nusselt number, hD

k

Rel = Reynolds number of liquid refrigerant, ρlvD

μl

v = velocity of refrigerant, m/s

L= length of the tube, m

x = dryness fraction

∆x = change in dryness fraction

6.4.3.2 Heat transfer coefficient on the fluid side

The heat transfer c

6.5 Design procedure for thermostatic expansion valve

Determine the evaporator temperature Te and the degree of superheat ∆Ts.

Find out the saturation pressures, psat (Te) and psat (Te + ∆Ts) of the refrigerant at Te and

Te + ∆Ts, either from standard refrigeration tables or using Antoine’s equation for R134a

psat = e14.41−2094

T−33.06

where psat is in kPa and T is in K

Find out the required adjustable spring pressure:

ps = psat(Te + ∆Ts) − psat(Te)

Based on the required deflection of spring, find out the area of the bellows:

Ab =Fsps